i have had my KSO's since they came out. i got them early, and in the states so i got them cheap. about 50 usd. i have since watched their popularity rise. their price also rose significantly.
sizing is important. sizing is not on shoe-size, but on foot length. you need to measure your feet (both of them) then refer to a sizing chart on the fivefingers website.

once you get them, putting them on and wearing them feels strange. everything about them is strange. throw away any expectations when you get them and just go with the flow. first-off i have mad grip in these. i can run up walls with them, do scissor climbs in my appartment hallway. oh, the soles are very marking, so your landlord may not be impressed.

you walk and run differently in them. if you can walk and run correctly, you will enjoy these. if you do not, you will learn or you will hurt. leading with a heel-strike hurts, and is not anatomically good for your body. lead with the ball of your foot for comfort and good feedback. if you get this you will find a good boost in your agility also.

the fivefingers have minimal insulation and no water proofness. if it is cold, your feet will feel the cold. if it is raining, your feet will get wet.
you dont wear socks with these, so they smell up super quick. i dont have sweaty or smelly feet, but i find i need to wash my fivefingers after a day of wear. i just chuck them in the washing machine with the rest of my clothes.

i dont use my fivefingers often. usually when i am feeling energised and in the mood for bouncing about. but when i do i enjoy them a lot.

Not too enthusiastic yet. I always buy size 10 and it always works, but in this case it feels like they're a bit large. With 5 finger shoes, your toes have to be in the pockets all the way, so large in this case means that the extra space is behind your heels, which makes it uncomfortable. Then again, the toe pockets feel pretty tight, so I'm not sure if I would fit into the next smaller version.

The Vibram Fivefingers concept is interesting: Vibram has been around for decades and so far they only made soles for other shoe manufacturers. This is their first own complete product on the market. The shoe is not much more that a protective layer of rubber (with a thin anti-sweat layer inside), so you do indeed get the barfoot feeling, without the risk of hurting your feet on sharp objects. Perfect first product for a company that used to make soles only - brilliant idea.

It looks pretty silly though, so if you don't like people staring at your feet, this is not for you.

Another drwaback is that you typically don't use socks with them, so it better be warm outside, as the shoes themselves are so thin that they hardly isolate.

I'll probably use them every now and then for a walk in summer, just because it feels good to walk (almost) barefoot, but otherwise they'll mostly stay in the closet.